(1) Field of the Invention
The present invention relates to a field emission electron source for emitting electrons from the cathode as well as relating to a fabrication process thereof. Detailedly, the invention relates to a field emission electron source which enables a low voltage, heavy current, stable configuration as well as relating to a fabrication process thereof.
(2) Description of the Prior Art
Recently, there has been marked progress in the fabrication technology for field emission electron sources that emit electrons in a high electric field, with the development of micro fabrication technology used in the field of integrated circuits or thin films. In particular, field emission electron source devices having highly miniaturized, field emission cold cathodes have been proposed. The emission field cold cathode of this type of electron source device is a most essential electron emission device, being one of the main parts constituting a micro-miniature electron tube of a triode type or micro- miniature electron gun.
FIGS. 1A to 1F show a fabrication method of a typical field emission electron source, which is disclosed, in Japanese Patent Application Laid-Open Hei 5 No.94,762.
First, an n-Si substrate 11 is thermally oxidized forming an ioxide film 15 on the surface thereof (FIG. 1A). Then, a circular cap 15a is formed after a photo process (FIG. 1B). When this substrate with the cap is dry etched, substrate 11 is etched with the portion beneath cap 15 being eroded from the underside so that a conical Si projection 11a is formed (FIG. 1C). Then, the substrate in this condition is thermally oxidized again forming an oxide film 12a (FIG. 1D). This oxide film 12a finally serves as an insulating film 12 between a cathode 10 and a gate 13 and also functions to keep the tip pointed. Next, gate electrode 13 is formed by oblique vapor deposition rotating around axis of rotation 16 (FIG. 1E), then the silicon oxide is etched by hydrofluoric acid to remove cap 15a and expose cathode tip 10 to thereby complete the intended field emission electron source (FIG. 1F).
FIG. 2 is an operational circuit diagram for operating this device. In a vacuum, an anode electrode 9 is arranged over cathode 10 of this field emission electron source with voltages applied between cathode 10 and gate 13 and between cathode 10 and anode 9. By this arrangement, the electric field between cathode 10 and gate 13 concentrates upon the pointed cathode end, causing cathode 10 to emit electrons forming a emitter portion, whereby electrons are drawn into the vacuum by the field emission tunnel effect.
These devices are very unstable at present, so that the obtainable current is low. If a large current is attempted, an excessive current flows through the cathode, finally, explosive meltdown and hence destruction could occur from Joule heat.
As an improvement against this, Japanese Patent Application Laid-Open Hei 3 No.220,337 offers deposition of a chemically stable, resistive material over the cathode surface. This method permits low voltage operation. However, if a heavy current is tried, the resistance of the resistive material generates heat so that there is a fear that the cathode tip""s temperature be raised and a meltdown occur. Therefore, this method involves a risk of the device being destroyed, resulting in unsuitability as a heavy-current device.
Japanese Patent Application Laid-Open Hei 5 No.274,998 has proposed formation of a gold thin film over the cathode tip surface using a focused ion beam. This method, however, needs large-scale equipment for generating the focused ion beam. Further, use of a precious metal such as gold sharply raises the fabrication cost of the device, making it difficult to realize a low-cost device.
U.S. Pat. No. 5,038,070 discloses a configuration shown in FIG. 3, in which three layers of metal films 51, 52 and 53 form cathodes 54 with hollows 55 therein. For fabrication, the three layers of metal films are formed on a substrate having concavities and then the substrate is removed to thereby form projections of the metal films. However, since the cathode is formed of thin films, even though it is of three layers, there is a concern that the pointed end of the cathode would rise in temperature and melt down if a heavy current flows. Therefore, this method also has a risk of the device being destroyed, resulting in unsuitability as a heavy-current device.
The present invention has been devised in order to solve the above problems and it is therefore an object of the present invention to provide a stable, field emission electron source and a fabrication method thereof, wherein a heavy current can be assured with a low voltage, by coating the cathode tip by inexpensive, physical vapor deposition or sputtering vapor deposition.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the present invention, a field emission electron source comprises:
a cathode made up of a substrate for emitting electrons;
a gate electrode for drawing electrons from the cathode; and
an insulating film for insulating the gate electrode and the cathode, and is characterized in that a metallic material having a lower resistance than that of the cathode substrate is coated as the first layer over the substrate surface, and then another material having a lower work function than that of the substrate is coated as the second layer over the surface of the first layer of metallic material.
In accordance with the second aspect of the present invention, the field emission electron source having the above first feature is characterized in that at least one selected from Mo, W, Ta, Nb, Hf, Zr and Ti is used as the first layer of metallic material and as least one selected from HfC, ZrC and TiC is used as the second layer of material having a low work function is used.
In accordance with the third aspect of the present invention, the field emission electron source having the above first feature is characterized in that the first layer of metallic material is formed with a film thickness of 5 nm or more.
In accordance with the fourth aspect of the present invention, the field emission electron source having the above second feature is characterized in that the first layer of metallic material is formed with a film thickness of 5 nm or more.
In accordance with the fifth aspect of the present invention, the field emission electron source having the above first feature is characterized in that the second layer of material having a low work function is formed with a film thickness of 3 nm or more.
In accordance with the sixth aspect of the present invention, the field emission electron source having the above second feature is characterized in that the second layer of material having a low work function is formed with a film thickness of 3 nm or more.
In accordance with the seventh aspect of the present invention, the field emission electron source having the above third feature is characterized in that the total film thickness of the first layer of metal material and the second layer of material having a low work function is equal to or smaller than 30 nm.
In accordance with the eighth aspect of the present invention, the field emission electron source having the above fourth feature is characterized in that the total film thickness of the first layer of metal material and the second layer of material having a low work function is equal to or smaller than 30 nm.
In accordance with the ninth aspect of the present invention, the field emission electron having the above fifth feature is characterized in that the total film thickness of the first layer of metal material and the second layer of material having a low work function is equal to or smaller than 30 nm.
In accordance with the tenth aspect of the present invention, the field emission electron source having the above sixth feature is characterized in that the total film thickness of the first layer of metal material and the second layer of material having a low work function is equal to or smaller than 30 nm.
In accordance with the eleventh aspect of the present invention, a fabrication method of a field emission electron source comprising: a cathode made up of a substrate for emitting electrons; a gate electrode for drawing electrons from the cathode; and an insulating film for insulating the gate electrode and the cathode, comprises the steps of:
coating a first layer of metallic material having a lower resistance than that of the cathode substrate, over the substrate surface; and
coating a second layer of material having a lower work function than that of the substrate, over the surface of the first layer of metallic material,
wherein the first layer of metallic material and second layer of material having a low work function are deposited by precisely directional film forming methods.
In accordance with the twelfth aspect of the present invention, a fabrication method of a field emission electron source comprising: a cathode made up of a substrate for emitting electrons; a gate electrode for drawing electrons from the cathode; and an insulating film for insulating the gate electrode and the cathode, comprises the steps of:
coating a first layer of metallic material having a lower resistance than that of the cathode substrate, over the substrate surface; and
coating a second layer of material having a lower work function than that of the substrate, over the surface of the first layer of metallic material,
wherein the first layer of metallic material and the second layer of material having a low work function are successively coated in the same vacuum.
In accordance with the thirteenth aspect of the present invention, the fabrication method of a field emission electron source having the above eleventh feature is characterized in that in the step of coating the first layer of metallic material and in the subsequent steps, silicon as the cathode substrate and the metallic material are inhibited from reacting with each other.
In accordance with the fourteenth aspect of the present invention, the fabrication method of a field emission electron source having the above twelfth feature is characterized in that in the step of coating the first layer of metallic material and in the subsequent steps, silicon as the cathode substrate and the metallic material are inhibited from reacting with each other.
In accordance with the fifteenth aspect of the present invention, the fabrication method.of a field emission electron source having the above thirteenth feature is characterized in that when the first layer of metallic material uses Mo, the film of the first layer of metallic material is formed at a temperature equal to or below 500xc2x0 C.
In accordance with the sixteenth aspect of the present invention, the fabrication method of a field emission electron source having the above fourteenth feature is characterized in that when the first layer of metallic material uses Mo, the film of the first layer of metallic material is formed at a temperature equal to or below 500xc2x0 C.
In accordance with the seventeenth aspect of the present invention, the fabrication method of a field emission electron source having the above thirteenth feature is characterized in that when the first layer of metallic material uses W, Ta or Nb, the film of the first layer of metallic material is formed at a temperature equal to or below 650xc2x0 C.
In accordance with the eighteenth aspect of the present invention, the fabrication method of a field emission electron source having the above fourteenth feature is characterized in that when the first layer of metallic material uses W, Ta or Nb, the film of the first layer of metallic material is formed at a temperature equal to or below 650xc2x0 C.
In accordance with the nineteenth aspect of the present invention, the fabrication method of a field emission electron source having the above thirteenth feature is characterized in that when the first layer of metallic material uses Hf, the film of the first layer of metallic material is formed at a temperature equal to or below 750xc2x0 C.
In accordance with the twentieth aspect of the present invention, the fabrication method of a field emission electron source having the above fourteenth feature is characterized in that when the first layer of metallic material uses Hf, the film of the first layer of metallic material is formed at a temperature equal to or below 750xc2x0 C.
In accordance with the twenty-first aspect of the present invention, the fabrication method of a field emission electron source having the above thirteenth feature is characterized in that when the first layer of metallic material uses Zr, the film of the first layer of metallic material is formed at a temperature equal to or below 700xc2x0 C.
In accordance with the twenty-second aspect of the present invention, the fabrication method of a field emission electron source having the above fourteenth feature is characterized in that when the first layer of metallic material uses Zr, the film of the first layer of metallic material is formed at a temperature equal to or below 700xc2x0 C.
In accordance with the twenty-third aspect of the present invention, the fabrication method of a field emission electron source having the above thirteenth feature is characterized in that when the first layer of metallic material uses Ti, the film of the first layer of metallic material is formed at a temperature equal to or below 600xc2x0 C.
In accordance with the twenty-fourth aspect of the present invention, the fabrication method of a field emission electron source having the above fourteenth feature is characterized in that when the first layer of metallic material uses Ti, the film of the first layer of metallic material is formed at a temperature equal to or below 600xc2x0 C.